Methods for making carboxylated cellulosic fibers

ABSTRACT

Methods for making carboxylated cellulosic fibers using the N-halo hindered cyclic amine compounds. N-halo hindered cyclic amine compounds react with a secondary oxidizing agent to provide a primary oxidizing agent that reacts with cellulosic fibers to provide carboxylated cellulosic fibers.

FIELD OF THE INVENTION

The present invention relates to methods for making carboxylatedcellulosic fibers, and compounds useful in the methods for makingcarboxylated cellulosic fibers.

BACKGROUND OF THE INVENTION

Cellulose is a carbohydrate consisting of a long chain of glucose units,all β-linked through the 1′-4 positions. Native plant cellulosemolecules may have upwards of 2200 anhydroglucose units. The number ofunits is normally referred to as degree of polymerization (D.P.). Someloss of D.P. inevitably occurs during purification. A D.P. approaching2000 is usually found only in purified cotton linters. Wood derivedcelluloses rarely exceed a D.P. of about 1700.

Chemical derivatives of cellulose have been commercially important foralmost a century and a half. Nitrocellulose plasticized with camphor wasthe first synthetic plastic and has been in use since 1868. A number ofcellulose ether and ester derivatives are presently commerciallyavailable and find wide use in many fields of commerce. Virtually allcellulose derivatives take advantage of the reactivity of the threeavailable hydroxyl groups (i.e., C2, C3, and C6). Substitution at thesegroups can vary from very low, about 0.01, to a maximum of 3. Amongimportant cellulose derivatives are cellulose acetate, used in fibersand transparent films; nitrocellulose, widely used in lacquers andgunpowder; ethyl cellulose, widely used in impact resistant toolhandles; methyl cellulose, hydroxyethyl, hydroxypropyl, and sodiumcarboxymethyl cellulose, water soluble ethers widely used in detergents,as thickeners in foodstuffs, and in papermaking.

Cellulose itself has been modified for various purposes. Cellulosefibers are naturally anionic in nature as are many papermakingadditives. A cationic cellulose is described in U.S. Pat. No. 4,505,775,issued to Harding et al. This cellulose has greater affinity for anionicpapermaking additives such as fillers and pigments and is particularlyreceptive to acid and anionic dyes. U.S. Pat. No. 5,667,637, issued toJewell et al., describes a low degree of substitution (D.S.)carboxyethyl cellulose which, along with a cationic resin, improves thewet to dry tensile and burst ratios when used as a papermaking additive.U.S. Pat. No. 5,755,828, issued to Westland, describes a method forincreasing the strength of articles made from crosslinked cellulosefibers having free carboxylic acid groups obtained by covalentlycoupling a polycarboxylic acid to the fibers.

For some purposes, cellulose has been oxidized to make it more anionicto improve compatibility with cationic papermaking additives and dyes.Various oxidation treatments have been used. Among these are nitrogendioxide and periodate oxidation coupled with resin treatment of cottonfabrics for improvement in crease recovery as suggested by Shet, R. T.and A. M. Nabani, Textile Research Journal, November 1981, pp. 740-744,.Earlier work by Datye, K. V. and G. M. Nabar, Textile Research Journal,July 1963, pp. 500-510, describes oxidation by metaperiodates anddichromic acid followed by treatment with chlorous acid for 72 hours or0.05 M sodium borohydride for 24 hours. Copper number was greatlyreduced by borohydride treatment and less so by chlorous acid. Carboxylcontent was slightly reduced by borohydride and significantly increasedby chlorous acid. The products were subsequently reacted withformaldehyde. Southern pine kraft springwood and summer wood fibers wereoxidized with potassium dichromate in oxalic acid. Luner, P., et al.,TAPPI 50(3):117-120, 1967. Handsheets made with the fibers showedimproved wet strength believed to be due to aldehyde groups. Pulps havealso been oxidized with chlorite or reduced with sodium borohydride.Luner, P., et al., Tappi 50(5):227-230, 1967. Handsheets made from pulpstreated with the reducing agent showed improved sheet properties overthose not so treated. Young, R. A., Wood and Fiber 10(2):112-119, 1978describes oxidation primarily by dichromate in oxalic acid to introducealdehyde groups in sulfite pulps for wet strength improvement in papers.Shenai, V. A. and A. S. Narkhede, Textile Dyer and Primer, May 20, 1987,pp. 17-22 describe the accelerated reaction of hypochlorite oxidation ofcotton yarns in the presence of physically deposited cobalt sulfide. Theauthors note that partial oxidation has been studied for the pasthundred years in conjunction with efforts to prevent degradation duringbleaching. They also discuss in some detail the use of 0.1 M sodiumborohydride as a reducing agent following oxidation. The treatment wasdescribed as a useful method of characterizing the types of reducinggroups as well as acidic groups formed during oxidation. The borohydridetreatment noticeably reduced copper number of the oxidized cellulose.Copper number gives an estimate of the reducing groups such as aldehydespresent on the cellulose. Borohydride treatment also reduced alkalisolubility of the oxidized product, but this may have been related to anapproximate 40% reduction in carboxyl content of the samples. Andersson,R., et al. in Carbohydrate Research 206: 340-346 (1990) describesoxidation of cellulose with sodium nitrite in orthophosphoric acid anddescribe nuclear magnetic resonance elucidation of the reactionproducts.

Davis, N. J., and S. L. Flitsch, Tetrahedron Letters 34(7): 1181-1184,1993 describe the use and reaction mechanism of2,2,6,6-tetramethylpiperidinyloxy free radical (TEMPO) with sodiumhypochlorite to achieve selective oxidation of primary hydroxyl groupsof monosaccharides. Following the Davis et al. paper this route tocarboxylation then began to be more widely explored. de Nooy, A. E. J.,et al., Receuil des Travaux Chimiques des Pays-Bas 113: 165-166, 1994reports similar results using TEMPO and hypobromite for oxidation ofprimary alcohol groups in potato starch and inulin. The following year,these same authors in Carbohydrate Research 269:89-98, 1995 reporthighly selective oxidation of primary alcohol groups in water solubleglucans using TEMPO and a hypochlorite/bromide oxidant.

WO 95/07303 (Besemer et al.) describes a method of oxidizing watersoluble carbohydrates having a primary alcohol group, using TEMPO withsodium hypochlorite and sodium bromide. Cellulose is mentioned inpassing in the background although the examples are principally limitedto starches. The method is said to selectively oxidize the primaryalcohol at C-6 to carboxylic acid group. None of the products studiedwere fibrous in nature.

WO 99/23117 (Viikari et al.) describes oxidation using TEMPO incombination with the enzyme laccase or other enzymes along with air oroxygen as the effective oxidizing agents of cellulose fibers, includingkraft pine pulps.

A year following the above noted Besemer publication, the same authors,in Cellulose Derivatives, Heinze, T. J. and W. G. Glasser, eds., 1996,Chap. 5, pp. 73-82, describe methods for selective oxidation ofcellulose to 2,3-dicarboxy cellulose and 6-carboxy cellulose usingvarious oxidants. Among the oxidants used were aperiodate/chlorite/hydrogen peroxide system, oxidation in phosphoricacid with sodium nitrate/nitrite, and with TEMPO and ahypochlorite/bromide primary oxidant. Results with the TEMPO system werepoorly reproduced and equivocal. In the case of TEMPO oxidation ofcellulose, little or none would have been expected to go into solution.The homogeneous solution of cellulose in phosphoric acid used for thesodium nitrate/sodium nitrite oxidation was later treated with sodiumborohydride to remove any carbonyl function present.

Chang, P. S. and J. F. Robyt, Journal of Carbohydrate Chemistry 15(7):819-830, 1996, describe oxidation of ten polysaccharides includingα-cellulose at 0 and 25° C. using TEMPO with sodium hypochlorite andsodium bromide. Ethanol addition was used to quench the oxidationreaction. The resulting oxidized α-cellulose had a water solubility of9.4%. The authors did not further describe the nature of theα-cellulose. It is presumed to have been a so-called dissolving pulp orcotton linter cellulose. Barzyk, D., et al., in Transactions of the 11thFundamental Research Symposium 2:893-907, 1997, note that carboxylgroups on cellulose fibers increase swelling and impact flexibility,bonded area and strength. They designed experiments to increase surfacecarboxylation of fibers. However, they ruled out oxidation to avoidfiber degradation and chose to form carboxymethyl cellulose in anisopropanol/methanol system.

Isogai, A. and Y. Kato, in Cellulose 5:153-164, 1998 describe treatmentof several native, mercerized, and regenerated celluloses with TEMPO toobtain water soluble and insoluble polyglucuronic acids. They note thatthe water soluble products had almost 100% carboxyl substitution at theC-6 site. They further note that oxidation proceeds heterogeneously atthe more accessible regions on solid cellulose.

Kitaoka, T., A. Isogai, and F. Onabe, in Nordic Pulp and Paper ResearchJournal 14(4):279-284, 1999, describe the treatment of bleached hardwoodkraft pulp using TEMPO oxidation. Increasing amounts of carboxyl contentgave some improvement in dry tensile index, Young's modulus, andbrightness, with decreases in elongation at breaking point and opacity.Other strength properties were unaffected. Retention of PAE-type wetstrength resins was somewhat increased. The products described did nothave any stabilization treatment after the TEMPO oxidation.

U.S. Pat. No. 6,379,494 describes a method for making stablecarboxylated cellulose fibers using a nitroxide-catalyzed process. Inthe method, cellulose is first oxidized by nitroxide catalyst to providecarboxylated as well as aldehyde and ketone substituted cellulose. Theoxidized cellulose is then stabilized by reduction of the aldehyde andketone substituents to provide the carboxylated fiber product.

Nitroxide-catalyzed cellulose oxidation occurs predominately at theprimary hydroxyl group on C-6 of the anhydroglucose moiety. In contrastto some of the other routes to oxidized cellulose, only very minoroxidation occurs at the secondary hydroxyl groups at C-2 and C-3.

In nitroxide oxidation of cellulose, primary alcohol oxidation at C-6proceeds through an intermediate aldehyde stage. In the process, thenitroxide is not irreversibly consumed in the reaction, but iscontinuously regenerated by a secondary oxidant (e.g., hypohalite) intothe nitrosonium (or oxyammonium) ion, which is the actual oxidant. Inthe oxidation, the nitrosonium ion is reduced to the hydroxylamine,which can be re-oxidized to the nitroxide. Thus, in the method, it isthe secondary oxidant (e.g., hypohalite) that is consumed. The nitroxidemay be reclaimed or recycled from the aqueous system.

The resulting oxidized cellulose product is an equilibrium mixtureincluding carboxyl and aldehyde substitution. Aldehyde substituents oncellulose are known to cause degeneration over time and under certainenvironmental conditions. In addition, minor quantities of ketone may beformed at C-2 and C-3 of the anhydroglucose units and these will alsolead to degradation. Marked degree of polymerization loss, fiberstrength loss, crosslinking, and yellowing are among the consequentproblems. Thus, to prepare a stabilized carboxylated product, aldehydeand ketone substituents formed in the oxidation step are reduced tohydroxyl groups in a stabilization step.

In addition to TEMPO, other nitroxide derivatives for makingcarboxylated cellulose fibers have been described. See, for example,U.S. Pat. No. 6,379,494 and WO 01/29309, Methods for Making CarboxylatedCellulose Fibers and Products of the Method.

A method of preparation of carboxylic acids or their salts by oxidationof primary alcohols using hindered N-chloro hindered cyclic amines andhypochlorite, in aqueous solutions or in mixed solvent systemscontaining ethyleneglycol dimethyl ether, diethyleneglycol dimethylether, triethyleneglycol dimethyl ether, toluene, acetonitrile,ethylacetate, t-butanol and other solvents is described in JP10130195,“Manufacturing Method of Carboxylic Acid and Its Salts”. Other oxidantsdescribed include chlorine, hypobromite, bromite, trichloro isocyanuricacid, tribromo isocyanuric acid, or combinations. This process has notbeen used to prepare stabilized carboxylated cellulosic pulp products.

Despite the advances made in the development of methods for makingcarboxylated cellulose pulps including catalytic oxidation systems,there remains a need for improved methods and catalysts for makingcarboxylated cellulose pulp. The present invention seeks to fulfillthese needs and provides further related advantages.

SUMMARY OF THE INVENTION

In one aspect of the present invention, compounds useful innitroxide-catalyzed oxidation of cellulose to make carboxylatedcellulosic fibers are provided. The compounds are N-halo hindered cyclicamine compounds that can be converted into the catalyst for oxidizingcellulose.

In another aspect, the present invention provides methods for makingcarboxylated cellulosic fibers using the N-halo hindered cyclic aminecompounds. In the method, the N-halo hindered cyclic amine compoundsreact with a secondary oxidizing agent to provide a primary oxidizingagent that reacts with cellulosic fibers to provide carboxylatedcellulosic fibers. In one embodiment, the carboxylated cellulosic fibersare further treated to provide stable carboxylated cellulosic fibers.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect of the present invention, N-halo hindered cyclic aminecompounds for use in making carboxylated cellulose pulp fibers areprovided. The N-halo hindered cyclic amine compounds are as effective asTEMPO and other related nitroxides in methods for making carboxylatedcellulose fibers.

The N-halo hindered cyclic amine compounds are fully alkylated at thecarbon atoms adjacent to the amino nitrogen atom (i.e., the N—Cl orN—Br) and have from 4 to 8 atoms in the ring. In one embodiment, theN-halo hindered cyclic amine compounds are six-membered ring compounds.In another embodiment, the N-halo hindered cyclic amine compounds arefive-membered ring compounds.

Representative N-halo hindered cyclic amine compounds useful in themethod of the invention for making carboxylated cellulose pulp fibersinclude Structures (I)-(VII).

Structure (I):

For Structure (I), R₁-R₄ can be C1-C6 straight-chain or branched alkylgroups, for example, methyl, ethyl, propyl, butyl, pentyl, or hexylgroups. Alternatively, R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl group, and R₃ and R₄ taken together can form afive- or six-carbon cycloalkyl group. The cycloalkyl group can befurther substituted with, for example, one or more C1-C6 alkyl groups orother substituents. X can be sulfur or oxygen. R₅ can be hydrogen,C1-C12 straight-chain or branched alkyl or alkoxy, aryl, aryloxy,benzyl, 2-dioxanyl, dialkyl ether, alkyl polyether, or hydroxyalkylgroup. Alternatively, R₅ can be absent and X can be hydrogen or a mirrorimage moiety to form a bipiperidinyl compound. A is a halogen, forexample, chloro or bromo. Representative compounds of Structure (I)include N-halo-2,2,6,6-tetramethylpiperidine;N,N′-dihalo-2,2,2′,2′,6,6,6′,6-octamethyl-4,4′-bipiperidine;N-halo-2,2,6,6-tetramethyl-4-hydroxypiperidine;N-halo-2,2,6,6-tetramethyl-4-methoxypiperidine; andN-halo-2,2,6,6-tetramethyl-4-benzyloxypiperidine.

Structure (II):

For Structure (II), R₁-R₄ can be C1-C6 straight-chain or branched alkylgroups, for example, methyl, ethyl, propyl, butyl, pentyl, or hexylgroups. Alternatively, R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl group, and R₃ and R₄ taken together can form afive- or six-carbon cycloalkyl group. The cycloalkyl group can befurther substituted with, for example, one or more C1-C6 alkyl groups orother substituents. X can be oxygen or sulfur. R₆ can be hydrogen, C1-C6straight-chain or branched alkyl groups. R₇ can be hydrogen, C1-C8straight-chain or branched alkyl groups, phenyl, carbamoyl, alkylcarbamoyl, phenyl carbamoyl, or C1-C8 acyl. A is a halogen, for example,chloro or bromo. Representative compounds of Structure (II) includeN-halo-2,2,6,6-tetramethyl-4-aminopiperidine andN-halo-2,2,6,6-tetramethyl-4-acetylaminopiperidine.

Structure (III):

For Structure (III), R₁-R₄ can be C1-C6 straight-chain or branched alkylgroups, for example, methyl, ethyl, propyl, butyl, pentyl, or hexylgroups. Alternatively, R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl group, and R₃ and R₄ taken together can form afive- or six-carbon cycloalkyl group. The cycloalkyl group can befurther substituted with, for example, one or more C1-C6 alkyl groups orother substituents. X can be oxygen, sulfur, NH, alkylamino (i.e.,NH-alkyl), dialkylamino, NOH, or NOR₁₀, where R₁₀ is a C1-C6straight-chain or branched alkyl group. A is a halogen, for example,chloro or bromo. A representative compound of Structure (III) isN-halo-2,2,6,6-tetramethylpiperidin-4-one.

Structure (IV):

For Structure (IV), R₁-R₄ can be C1-C6 straight-chain or branched alkylgroups, for example, methyl, ethyl, propyl, butyl, pentyl, or hexylgroups. Alternatively, R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl group, and R₃ and R₄ taken together can form afive- or six-carbon cycloalkyl group. The cycloalkyl group can befurther substituted with, for example, one or more C1-C6 alkyl groups orother substituents. X can be oxygen, sulfur, alkylamino (i.e., N—R₁₀),or acylamino (i.e., N—C(═O)—R₁₀), where R₁₀ is a C1-C6 straight-chain orbranched alkyl group. A is a halogen, for example, chloro or bromo. Arepresentative compound of Structure (IV) isN-halo-3,3,5,5-tetramethylmorpholine.

Structure (V):

For Structure (V), R₁-R₄ can be C1-C6 straight-chain or branched alkylgroups, for example, methyl, ethyl, propyl, butyl, pentyl, or hexylgroups. Alternatively, R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl group, and R₃ and R₄ taken together can form afive- or six-carbon cycloalkyl group. The cycloalkyl group can befurther substituted with, for example, one or more C1-C6 alkyl groups orother substituents. A is a halogen, for example, chloro or bromo. Arepresentative compound of Structure (V) isN-halo-3,4-dehydro-2,2,6,6,-tetramethylpiperidine.

Structure (VI):

For Structure (VI), R₁-R₄ can be C1-C6 straight-chain or branched alkylgroups, for example, methyl, ethyl, propyl, butyl, pentyl, or hexylgroups. Alternatively, R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl group, and R₃ and R₄ taken together can form afive- or six-carbon cycloalkyl group. The cycloalkyl group can befurther substituted with, for example, one or more C1-C6 alkyl groups orother substituents. X can be methylene (i.e., CH₂), oxygen, sulfur, oralkylamino. R₈ and R₉ can be independently selected from C1-C6straight-chain or branched alkyl groups, for example, methyl, ethyl,propyl, butyl, pentyl, or hexyl groups. Alternatively, R₈ and R₉ takentogether can form a five- or six-membered ring, which can be furthersubstituted with, for example, one or more C1-C6 alkyl groups or othersubstituents. A is a halogen, for example, chloro or bromo.Representative compounds of Structure (VI) include N-halo-4-piperidoneketals, such as ethylene, propylene, glyceryl, and neopentyl ketals.Representative compounds of Structure (VI) includeN-halo-2,2,6,6-tetramethyl-4-piperidone ethylene ketal,N-halo-2,2,6,6-tetramethyl-4-piperidone propylene ketal,N-halo-2,2,6,6-tetramethyl-4-piperidone glyceryl ketal, andN-halo-2,2,6,6-tetramethyl-4-piperidone neopentyl ketal.

Structure (VII):

For Structure (VII), R₁-R₄ can be C1-C6 straight-chain or branched alkylgroups, for example, methyl, ethyl, propyl, butyl, pentyl, or hexylgroups. Alternatively, R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl group, and R₃ and R₄ taken together can form afive- or six-carbon cycloalkyl group. The cycloalkyl group can befurther substituted with, for example, one or more C1-C6 alkyl groups orother substituents. X can be methylene, oxygen, sulfur, NH, (i.e.,N—R₁₀), or acylamino (i.e., N—C(═O)—R₁₀), where R₁₀ is a C1-C6straight-chain or branched alkyl group. A is a halogen, for example,chloro or bromo. A representative compound of Structure (VII) isN-halo-2,2,5,5-tetramethylpyrrolidine.

In general, the N-halo hindered cyclic amine compounds noted above canbe prepared by chlorination or bromination of the corresponding aminecompounds. Standard methods of preparing N-halo hindered amines aredescribed in the literature. See, for example, (1) Rigo, A., et al.,Inorganica Chimica Acta 35:61-164, 1979; (2) Fujiwara, M., et al., J.Am. Oil. Chem. Soc. 72(1):97-103, 1995; and (3) U.S. Pat. No. 4,931,562.For example, 4-hydroxy-2,2,6,6-tetramethyl-N-bromopiperidine can beprepared according to the procedure described in Rigo, A., et al.,Inorganica Chimica Acta 35:61-164, 1979; andchloro-4-N,N-bis(2-hydroxyethyl)amino-2,2,6,6-tetramethylpiperidine canbe prepared according to the procedure described in U.S. Pat. No.4,931,562. Other N-chloro hindered amine compounds described herein canbe prepared by similar synthetic and purification methods.

The invention provides a method for making carboxylated cellulose pulpfibers using an N-halo hindered cyclic amine compound in aqueous mediaunder heterogeneous conditions. In the method, the N-halo hinderedcyclic amine compound reacts with a secondary oxidizing agent (e.g.,chlorine dioxide, peracids, hypochlorites, chlorites, ozone, hydrogenperoxide, potassium superoxide) to provide a primary oxidizing agentthat reacts with cellulose pulp fibers to provide carboxylated cellulosepulp fibers. In one embodiment, the carboxylated cellulosic fibers arefurther treated to provide stable carboxylated cellulosic fibers. In themethod, under basic pH conditions and in the presence of a secondaryoxidizing agent, the primary oxidizing agent is generated from theN-halo hindered cyclic amine compound. In one embodiment, thecarboxylated cellulosic fibers are further treated to provide stablecarboxylated cellulosic fibers.

As noted above, in one embodiment, the method for making carboxylatedcellulose pulp fibers includes two steps: (1) a carboxylation step; and(2) a stabilization step.

In the carboxylation step, cellulose pulp fibers are oxidized (i.e.,carboxylated) under basic pH conditions and in the presence of asecondary oxidizing agent, such as chlorine dioxide, hypochlorite,peracids, or certain metal ions, with a catalytically active species(e.g., an oxammonium ion) generated from a N-halo hindered cyclic aminecompound described above.

The carboxylation reaction generally takes place at a temperature fromabout 20° C. to about 90° C. The N-halo hindered cyclic amine compoundis present in an amount from about 0.002% to about 0.25% by weight basedon the total weight of the pulp. The secondary oxidizing agent ispresent in an amount from about 0.1 to about 10% by weight based on thetotal weight of the pulp. Reaction times for carboxylating the pulprange from about 5 seconds to about 10 hours, depending upon reactiontemperature and the amount of N-halo hindered cyclic amine compound andsecondary oxidizing agent.

Suitable secondary oxidizing agents include hypohalites, chlorinedioxide, and peracids such as Caro's Acid. In one embodiment, thehypohalite is sodium hypochlorite (NaOCl). Sodium hypochlorite isinexpensive and readily available as a stable aqueous solution withabout 4-10% NaOCl w/v and can be made in situ by bubbling chlorine gasinto a solution of NaOH. Admixture of NaOCl with sodium bromide (NaBr)can accelerate the oxidation reaction. In one embodiment of the method,this combination is used (e.g., about 3 parts by weight NaBr to 4 partsof NaOCl is suitable). The amount of NaOCl is in the range of about 0.8to about 6.5 g/L of pulp slurry, preferably about 1.1 to about 1.4 g/L.The amount of NaOCl based on cellulose will be within the range of about0.5 to about 35% by weight, preferably about 1.3 to about 10.5% byweight. The amount of NaOCl will depend on the amount of carboxylationdesired. The pH during oxidation should generally be maintained withinthe range of about 6.5 to 11, preferably about 6.5 to 10, and mostpreferably about 7.5 to 9.5. The oxidation reaction will proceed athigher and lower pH values, but at lower efficiencies.

A proprietary composition sold as STABREX, available from Nalco ChemicalCo., Chicago, Ill., may be used in place of the hypochlorite oxidizingagent. STABREX is sold as an aqueous stabilized highly alkaline solutionof a bromine-containing composition having 1-5% NaOH, a minimum pH of13, and is a latent source of hypobromite. The composition contains astabilizer believed to be a sulfonated nitrogen-containing compound.STABREX is useful where environmental or other considerations mightdictate against the use of chlorine-based materials.

It will be understood that in accordance with usual reaction kinetics,oxidation will proceed at a higher rate with increased concentrations ofoxidizing agents and at higher temperature. Reaction at lowertemperatures (e.g., 0-10° C.) is preferred to reduce cellulose D.P.degradation. However, the reaction may also be carried out at highertemperatures to produce products having a D.P. higher than 600.

Following oxidation, the cellulose can be washed to remove any residualchemicals and may then be dried or further processed. If maximumstability and D.P. retention is desired, the oxidized product isre-slurried in water for treatment with a stabilizing agent. Thestabilizing agent may either be a reducing agent or an oxidizing agent.A preferred reducing agent is preferably an alkali metal borohydride.Sodium borohydride (NaBH₄) is preferred from the standpoint of cost andavailability. However, other borohydrides, such as lithium borohydride(LiBH₄), or alkali metal cyanoborohydrides, such as sodiumcyanoborohydride (NaBH₃CN), are also suitable. NaBH₄ may be mixed withLiCl to form a useful reducing agent. When NaBH₄ is used for reduction,it should be present in an amount between about 0.1 and 100 g/L; in oneembodiment, about 0.25 to about 5 g/L; and, in another embodiment, about0.5 to about 2 g/L. Based on cellulose the amount of reducing agent issuitably in the range of about 0.1% to 4% by weight, preferably about 1to about 3%. Reduction may be carried out at room or higher temperaturefor a time between 10 minutes and 10 hours, preferably from about 30minutes to 2 hours.

Alkali metal chlorites are preferred oxidizing agents used asstabilizers, with sodium chlorite being preferred because of the costfactor. Other compounds that may serve equally well as oxidizers includepermanganates, chromic acid, bromine, and silver oxide. A combination ofchlorine dioxide and hydrogen peroxide is also a suitable oxidizer whenused at the pH range designated for sodium chlorite. Oxidation usingsodium chlorite may be carried out at a pH in the range of about 1.5 toabout 6, preferably from about 2 to about 4, at temperatures betweenabout 25 to about 90° C. for times from about 5 minutes to 50 hours,preferably from about 10 minutes to about 2 hours. One factor thatfavors oxidizing agents as opposed to reducing agents is that aldehydegroups on the oxidized cellulose are converted to additional carboxylgroups, thus resulting in a more highly carboxylated product. Thesestabilizing oxidizers are referred to as “tertiary oxidizers” todistinguish them from the N-halo hindered cyclic amine(primary)/chlorine dioxide, peracids including Caro's acid,hypochlorites, chlorites, ozone, hydrogen peroxide (secondary)oxidizers. The tertiary oxidizer is used in a molar ratio of about 1 to15 times the presumed aldehyde content of the oxidized cellulose,preferably about 5 to 10 times. In a more convenient way of measuringthe required tertiary oxidizer needed, the referred sodium chloriteusage should fall within about 0.001 or sodium chlorite/g fiber to 0.2g/g, preferably 0.01 to 0.09 g/g, the chlorite being calculated on a100% active material basis.

After stabilization is completed, the cellulose is again washed and maybe dried if desired. Alternatively, the carboxyl substituents may beconverted to other cationic forms in addition to hydrogen or sodium, forexample, calcium, magnesium, or ammonium.

The method of the invention provides a cellulosic fiber having anincrease in carboxyl substitution compared to the starting cellulosicfibers of at least about 2 meq/100 g. In one embodiment, the methodprovides a cellulosic fiber having an increase in carboxyl substitutioncompared to the starting cellulosic fibers of at least about 5 meq/100g. Because carboxylation occurs predominately at C-6 (i.e., —CH₂OH) ofthe anhydroglucose moiety of cellulose, the carboxylated fibers producedby the method include C-6 carboxyl groups (i.e., glucuronic acidgroups).

One particular advantage of the process is that all reactions arecarried out in an aqueous medium to yield a product in which thecarboxylation is primarily located on the fiber surface. This conveyshighly advantageous properties for papermaking.

The carboxylated fiber produced by the method of the invention is highlyadvantageous as a papermaking furnish, either by itself or inconjunction with conventional fiber. The carboxylated fiber may be usedin an amount from about 0.5 to about 100% of the papermaking furnish.The carboxylated fiber is especially useful in admixture with recycledfiber to add strength. The carboxylated fiber's increased number ofanionic sites should serve to ionically hold significantly largeramounts of cationic papermaking additives than untreated fiber. Theseadditives can include wet strength resins, sizing chemical emulsions,filler and pigment retention aids, charged filler particles, dyes, andthe like. Carboxylated pulps do not hornify (or irreversibly collapse)as much on drying and are a superior material when recycled and swellmore on rewetting, take less energy to refine, and give higher sheetstrength.

The following examples are provided for the purpose of illustrating, notlimiting, the invention.

EXAMPLE Example 1 Representative Method for Making CarboxylatedCellulose Pulp

In this example, a representative method for making carboxylatedcellulose pulp is described.

Fully bleached never dried cellulose pulp 10 g OD (35.7 g wet) wastreated with 250 mL of 0.7% chlorine dioxide solution containing 0.250 gof 1-chloro-2,2,6,6-tetramethyl-4-piperidone ethylene ketal. The pH wasadjusted to 8.5 with sodium carbonate and the slurry was heated in aplastic bag at 70° C. for 45 minutes. The pH was adjusted to 3.5 withdilute sulfuric acid and the slurry treated with 40 mL of hydrogenperoxide (30%) solution and 100 mL of 0.7% chlorine dioxide. The pulpslurry was heated at 70° C. for 45 minutes. The pH of the slurry wasthen adjusted to 9.5 with 50 mL of aqueous sodium carbonate solution andfiltered.

The filtered pulp slurry was washed thoroughly with de-ionized water.The extent of the pulp carboxylation was determined by titrationmodified TAPPI method and was found to be 17.0 meq/100 g.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A method for making carboxylated cellulosic fibers, comprising:reacting an N-halo hindered cyclic amine compound with secondaryoxidizing agent to provide a primary oxidizing agent; and contacting theprimary oxidizing agent with cellulosic fibers to provide carboxylatedcellulosic fibers.
 2. The method of claim 1, wherein the N-halo hinderedcyclic amine compound comprises a cyclic amine compound that is fullyalkylated at the carbon atoms adjacent to the amino nitrogen atom. 3.The method of claim 1, wherein the N-halo hindered cyclic amine compoundcomprises a cyclic amine compound having from 4 to 8 atoms in the ring.4. The method of claim 1, wherein the N-halo hindered cyclic aminecompound comprises a five-membered ring compound.
 5. The method of claim1, wherein the N-halo hindered cyclic amine compound comprises asix-membered ring compound.
 6. The method of claim 1, wherein the N-halohindered cyclic amine compound has the structure:

wherein R₁-R₄ are independently at least one of C1-C6 straight-chain orbranched alkyl, or R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl, or R₃ and R₄ taken together can form a five- orsix-carbon cycloalkyl; X is oxygen or sulfur; R₅ is at least one ofhydrogen, C1-C12 straight-chain or branched alkyl or alkoxy, aryl,aryloxy, benzyl, 2-dioxanyl, dialkyl ether, alkyl polyether, orhydroxyalkyl; and A is at least one of chloro or bromo.
 7. The method ofclaim 1, wherein the N-halo hindered cyclic amine compound has thestructure:

wherein R₁-R₄ are independently at least one of C1-C6 straight-chain orbranched alkyl, or R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl, or R₃ and R₄ taken together can form a five- orsix-carbon cycloalkyl; R₆ is at least one of C1-C6 straight-chain orbranched alkyl; R₇ is at least one of hydrogen, C1-C8 straight-chain orbranched alkyl, phenyl, carbamoyl, alkyl carbamoyl, phenyl carbamoyl, orC1-C8 acyl; and A is at least one of chloro or bromo.
 8. The method ofclaim 1, wherein the N-halo hindered cyclic amine compound has thestructure:

wherein R₁-R₄ are independently at least one of C1-C6 straight-chain orbranched alkyl, or R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl, or R₃ and R₄ taken together can form a five- orsix-carbon cycloalkyl; X is at least one of oxygen, sulfur, NH,alkylamino, dialkylamino, NOH, or NOR₁₀, wherein R₁₀ is a C1-C6straight-chain or branched alkyl; and A is at least one of chloro orbromo.
 9. The method of claim 1, wherein the N-halo hindered cyclicamine compound has the structure:

wherein R₁-R₄ are independently at least one of C1-C6 straight-chain orbranched alkyl, or R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl, or R₃ and R₄ taken together can form a five- orsix-carbon cycloalkyl; X is at least one of oxygen, sulfur, N—R₁₀, orN—C(═O)—R₁₀, wherein R₁₀ is a C1-C6 straight-chain or branched alkyl;and A is at least one of chloro or bromo.
 10. The method of claim 1,wherein the N-halo hindered cyclic amine compound has the structure:

wherein R₁-R₄ are independently at least one of C1-C6 straight-chain orbranched alkyl, or R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl, or R₃ and R₄ taken together can form a five- orsix-carbon cycloalkyl; and A is at least one of chloro or bromo.
 11. Themethod of claim 1, wherein the N-halo hindered cyclic amine compound hasthe structure:

wherein R₁-R₄ are independently at least one of C1-C6 straight-chain orbranched alkyl, or R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl, or R₃ and R₄ taken together can form a five- orsix-carbon cycloalkyl; X is at least one of methylene, oxygen, sulfur,or alkylamino; R₈ and R₉ are independently at least one of C1-C6straight-chain or branched alkyl groups, or R₈ and R₉ taken together canform a five- or six-membered ring; and A is at least one of chloro orbromo.
 12. The method of claim 1, wherein the N-halo hindered cyclicamine compound has the structure:

wherein R₁-R₄ are independently at least one of C1-C6 straight-chain orbranched alkyl, or R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl, or R₃ and R₄ taken together can form a five- orsix-carbon cycloalkyl; X is at least one of methylene, oxygen, sulfur,NH, N—R₁₀, or N—C(═O)—R₁₀, wherein R₁₀ is a C1-C6 straight-chain orbranched alkyl; and A is at least one of chloro or bromo.
 13. The methodof claim 1, wherein the primary oxidizing agent is at least one ofchlorine dioxide, a peracid, hydrogen peroxide, ozone, or a hypohalite.14. The method of claim 13, wherein the hypohalite comprises sodiumhypochlorite.
 15. The method of claim 1, wherein the carboxylated fiberscomprises C-6 carboxyl groups.
 16. The method of claim 1 furthercomprising stabilizing the carboxylated fibers.
 17. The method of claim16, wherein stabilizing the carboxylated fibers comprises treating thecarboxylated fibers with a reducing agent.
 18. The method of claim 17,wherein the reducing agent is at least one of sodium borohydride,lithium borohydride, or sodium cyanoborohydride.
 19. The method of claim16, wherein stabilizing the carboxylated fibers comprises treating thecarboxylated fibers with an oxidizing agent.
 20. The method of claim 19,wherein the oxidizing agent is at least one of sodium chlorite, chlorinedioxide, or hydrogen peroxide.
 21. A method for making stablecarboxylated cellulosic fibers, comprising: reacting an N-halo hinderedcyclic amine compound with secondary oxidizing agent to provide aprimary oxidizing agent; contacting the primary oxidizing agent withcellulosic fibers to provide carboxylated cellulosic fibers; andtreating the carboxylated cellulosic fibers with a stabilizing agent toprovide stabilized carboxylated cellulosic fibers.
 22. The method ofclaim 21, wherein the N-halo hindered cyclic amine compound is anN-chloro-2,2,6,6-tetramethyl-4-piperidone ketal.
 23. The method of claim21, wherein the N-halo hindered cyclic amine compound isN-chloro-2,2,6,6-tetramethyl-4-piperidone ethylene glycol ketal.
 24. Themethod of claim 21, wherein the secondary oxidizing agent is at leastone of chlorine dioxide, a peracid, hydrogen peroxide, or ozone.
 25. Themethod of claim 21, wherein the stabilizing agent is a chlorite.
 26. Acompound having the formula:

wherein X is oxygen or sulfur; R₁-R₄ are independently at least one ofC1-C6 straight-chain or branched alkyl, or R₁ and R₂ taken together canform a five- or six-carbon cycloalkyl, or R₃ and R₄ taken together canform a five- or six-carbon cycloalkyl; R₅ is at least one of hydrogen,C1-C12 straight-chain or branched alkyl or alkoxy, aryl, aryloxy,benzyl, 2-dioxanyl, dialkyl ether, alkyl polyether, or hydroxyalkyl; andA is at least one of chloro or bromo.
 27. A compound having the formula:

wherein R₁-R₄ are independently at least one of C1-C6 straight-chain orbranched alkyl, or R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl, or R₃ and R₄ taken together can form a five- orsix-carbon cycloalkyl; R₆ is at least one of C1-C6 straight-chain orbranched alkyl; R₇ is at least one of hydrogen, C1-C8 straight-chain orbranched alkyl, phenyl, carbamoyl, alkyl carbamoyl, phenyl carbamoyl, orC1-C8 acyl; and A is at least one of chloro or bromo.
 28. A compoundhaving the formula:

wherein X is at least one of oxygen, sulfur, NH, alkylamino,dialkylamino, NOH, or NOR₁₀, wherein R₁₀ is a C1-C6 straight-chain orbranched alkyl; R₁-R₄ are independently at least one of C1 —C6straight-chain or branched alkyl, or R₁ and R₂ taken together can form afive- or six-carbon cycloalkyl, or R₃ and R₄ taken together can form afive- or six-carbon cycloalkyl; and A is at least one of chloro orbromo.
 29. A compound having the formula:

wherein X is at least one of oxygen, sulfur, N—R₁₀, or N—C(═O)—R₁₀,wherein R₁₀ is a C1-C6 straight-chain or branched alkyl; R₁-R₄ areindependently at least one of C1-C6 straight-chain or branched alkyl, orR₁ and R₂ taken together can form a five- or six-carbon cycloalkyl, orR₃ and R₄ taken together can form a five- or six-carbon cycloalkyl; andA is at least one of chloro or bromo.
 30. A compound having the formula:

wherein R₁-R₄ are independently at least one of C1-C6 straight-chain orbranched alkyl, or R₁ and R₂ taken together can form a five- orsix-carbon cycloalkyl, or R₃ and R₄ taken together can form a five- orsix-carbon cycloalkyl; and A is at least one of chloro or bromo.
 31. Acompound having the formula:

wherein X is at least one of methylene, oxygen, sulfur, or alkylamino;R₁-R₄ are independently at least one of C1-C6 straight-chain or branchedalkyl, or R₁ and R₂ taken together can form a five- or six-carboncycloalkyl, or R₃ and R₄ taken together can form a five- or six-carboncycloalkyl; R₈ and R₉ are independently at least one of C1-C6straight-chain or branched alkyl groups, or R₈ and R₉ taken together canform a five- or six-membered ring; and A is at least one of chloro orbromo.
 32. A compound having the formula:

wherein X is at least one of methylene, oxygen, sulfur, NH, N—R₁₀, orN—C(═O)—R₁₀, wherein R₁₀ is a C1-C6 straight-chain or branched alkyl;R₁-R₄ are independently at least one of C1-C6 straight-chain or branchedalkyl, or R₁ and R₂ taken together can form a five- or six-carboncycloalkyl, or R₃ and R₄ taken together can form a five- or six-carboncycloalkyl; and A is at least one of chloro or bromo.